The present invention generally relates to an exhaust gas purification and, more particularly, to an exhaust gas purifying apparatus for an automotive vehicle engine of a type having a plurality of engine cylinders.
U.S. Pat. No. 3,908,371, patented on Sept. 30, 1975, discloses an exhaust gas purifying apparatus for an automotive vehicle, which utilizes a dual-catalyst exhaust treatment system including a reducing catalyst for removing an excess of oxygen and reducing nitrogen oxides in the exhaust gases and an oxidizing catalyst for oxidizing carbon monoxide and hydrocarbons in the exhaust gases. More specifically, according to the above mentioned U.S. patent, a plurality of exhaust ducts respectively leading from exhaust outlets of the corresponding engine cylinders are merged into a single exhaust manifold having reducing and oxidizing catalyst units disposed therein so that the exhaust gases emitted from the engine cylinders subsequent to combustion of an air-fuel mixture within such engine cylinders can, after having been joined together, pass through the reducing catalyst unit and then through the oxidizing catalyst unit prior to such exhaust gases being discharged to the atmosphere. In order to enhance oxidization of carbon monoxide and hydrocarbons in the exhaust gases which have passed through the reducing catalyst unit, the purifying apparatus of the above mentioned United States patent further comprises a secondary air supply means including an engine operated pump for introducing secondary air into a portion of the exhaust manifold between the reducing catalyst unit and the oxidizing catalyst unit.
In the prior art dual-catalyst purifying apparatus having the construction described above, the engine operated pump for introducing the secondary air into the manifold portion between the reducing and oxidizing catalyst units is essential. The reason for this is obviously rooted in the fact that all of the exhaust ducts respectively leading from the individual engine cylinders are merged into the single exhaust manifold at a position upstream of the reducing catalyst unit in terms of the direction of flow of the exhaust gases towards the atmosphere. More specifically, as is well known to those skilled in the art, automobile exhaust gases successively emitted from the engine cylinders generally flow through an exhaust manifold towards the atmosphere in a substantially pulsating manner with the pressure inside the exhaust manifold alternately increasing and decreasing, because of the different times of firing of air-fuel mixtures supplied into the respective engine cylinders.
While this is well known to those skilled in the art, if all of the exhaust gases successively emitted from the individual engine cylinder are allowed to pass through the reducing catalyst unit such as in the prior art dual-catalyst purifying apparatus, the pulsating characteristic of flow of the exhaust gases through the exhaust manifold downstream of the reducing catalyst unit is so weakened that the prior art dual-catalyst purifying apparatus cannot make use of such pulsating characteristic in supplying the secondary air into the exhaust manifold portion downstream of the reducing catalyst unit and upstream of the oxidizing catalyst unit, and an external pumping means, such as the engine operated pump, is accordingly required.
The employment of the engine operated pump results in a substantial increase in the cost of manufacture of the purifying apparatus which in turn reflects upon the cost of manufacture of an automotive vehicle equipped with such purifying apparatus. In addition, since the engine operated pump is driven from a power output shaft of the engine, the power output shaft is unnecessarily loaded which constitutes one of the causes of reduction in the power output of the engine as a whole.